1. Field of the Invention
The present invention generally relates to connectors for optical fiber waveguides, and more particularly to a keying element for a fiber distributed data interface connector.
2. Description of the Prior Art
The use of optical fibers for high speed communication and data transmission via optical signals has become well established. There are already hundreds of thousands of miles of optical fiber in use today. As with copper wires, it is necessary to provide connections between optical fibers at various locations in the distribution system, whether during the installation of new fibers, or during the repair or replacement of existing fibers. It has, therefore, become imperative to provide optical fiber connectors which may be inexpensively manufactured, as well as easily assembled in the field to provide connections between existing fibers and electro-optical devices. As used herein, the term "connector" refers to a detachable and refastenable device, as opposed to a "splice" which usually provides a permanent connection.
There are several conventional optical fiber connector designs, including those commonly referred to as ST, SC, FC, D4, SMA, and biconic connectors. Each of these designs are simplex, i.e., they are used to connect a single pair of fibers, although they may be adapted for duplex use. Other connectors have been specifically designed for duplex connections, including those shown in U.S. Pat. Nos. 4,779,950, 4,979,792 and 5,016,968. The connector design shown in the latter two patents is referred to as FDDI, for Fiber Distributed Data Interface, and is used, among other things, for data transmission and reception between computer systems, particularly local area networks. A typical FDDI connector 1 is depicted in FIG. 1. The prior art FDDI connector 1 includes a body or plug 2 having a hole at one end for receiving the fiber cable 3. Cable 3 breaks out inside plug 2 into two separate optical fibers which are attached to and terminate in two ferrules 4. Plug 2 may mate with one of several different receptacles, including an active device receptacle, a transceiver adaptor, a dual ST coupling, or an FDDI-to-FDDI coupling. Latch members 5 releasably secure plug 2 in the given receptacle.
Each of these receptacles typically includes a projection or spline which acts as a key or polarizing element to insure that the particular receptacle being used is compatible with the overall data transmission system. In this regard, the American National Standards Institute (ANSI) has established several standards for FDDI receptacles, including the keying formats. The keying considerations are the subject of ANSI standard X3T9.5/84-48 (see FIGS. 4-5 of that standard, "receptacle keying detail"). Plug 2 includes a groove 6 formed along one surface thereof, designed to accommodate the projection or spline. There are four basic keying formats under this standard, denoted as A, B, M and S connections. Each format is accomplished by providing a narrow channel in one of three lateral positions along groove 6 (actually, by providing a corresponding ridge or spline in the receptacle), except for the "S" connection ("slave") which is formed by having a channel which is the full width of groove 6, i.e., an "S" receptacle has a wider spline which fills the entire groove. As viewed from the front of plug 2 (i.e., looking from the end near ferrules 4 toward cable 3), the channels for the other three formats are located as follows. For an "A" connection (denoting a main ring trunk with primary in, secondary out), the channel is located at the right side of groove 6. For a "B" connection (denoting a main ring trunk with secondary in, primary out), the channel is located at the left side of groove 6. For an "M" connection (denoting a master connection of a concentrator), the channel is located at the center of groove 6.
In order to avoid the necessity of molding four different plugs for these four keying formats, the prior art connector 1 provides the groove 6 (which is wide enough to accommodate a spline located in any of the three lateral positions) in conjunction with three different keying elements which fit into a hole 7 in groove 6. Each keying element has a shaft which fits into hole 7, and a cross-member which has the narrow channel in the appropriate lateral location. In FIG. 1, two such keying elements are shown, "M" keying element 8, and "A" keying element 9; the "S" format is inherent by simply not placing any element into hole 7. The prior art keying elements are used interchangeably by removing one element and replacing it with the element appropriate to that particular connection. Several problems have arisen, however, in the use of such replaceable keying elements. First of all, since the manufacturer does not know which of the formats will be needed by the user, all three of the elements must be provided. Secondly, since the elements are completely removable from plug 2, it is very easy to lose them. A related problem is the need to store the keying elements that are not presently being used, in case the user desires to later change the connection format. In the prior art design, this problem was addressed by molding storage compartments into the dust cap of the plug (the dust cap being attached to cable 3 by a tether). But the provision of the storage compartments and tether unnecessarily adds cost to the FDDI connector. Finally, the provision of three different keying elements, requiring three different molds, also adds cost to the overall system. It would, therefore, be desirable and advantageous to devise an alternative keying element which would overcome the foregoing disadvantages.